1. Field of the Invention
This invention relates to curable fluorinated silicone elastomeric compositions and method of production and use.
2. Description of the Prior Art
Safford and Bueche teach in U.S. Pat. No. 2,710,290, issued June 7, 1955, that a composition of matter comprising an organopolysiloxane convertable by heat to the cured, solid, elastic state, having monovalent hydrocarbon radicals, and a minor proportion of solid polytetrafluoroethylene can be mixed and then cured to an elastomer with improved physical properties, particularly tear strength. They teach their composition appears to have better hydrocarbon oil resistance and resistance to aromatic solvents than does the same composition without the polytetrafluoroethylene.
Polmanteer in Canadian Pat. No. 567,259, issued Dec. 9, 1958, discloses a composition comprising an organopolysiloxane, having monovalent hydrocargon radicals, polyfluorinated aliphatic monovalent hydrocarbon radicals and halogenated aromatic monovalent hydrocarbon radicals; from 5 to 100 parts by weight, based upon 100 parts by weight of the organopolysiloxane, of a powdered fluorocarbon and a vulcanizing agent. When the compositions were vulcanized, elastomeric materials were produced which had high tear resistance combined with low compression set. One of the fluorocarbon materials found suitable was polytetrafluoroethylene. The compositions were useful in the formation of gaskets and for other uses requiring combinations of high tear resistance, low compression set, excellent thermal stability, and chemcial inertness.
Konkle and Talcott teach in U.S. Pat. No. 2,927,908 issued Mar. 8, 1960, that the addition of polytetrafluoroethylene to fluorinated organopolysiloxane rubbers improves the tensile strength and tear resistance of the rubber. They teach that it was already known that small quantities of polytetrafluoroethylene in silicone rubber stocks improved the tear strength. At a fairly low proportion of polytetrafluoroethylene to previously employed organopolysiloxane stock, The tensile strength of the stock reached a peak, after which subsequent additions of polytetrafluoroethylene decreased the tensile strength, while still more was found to be incompatible. Konkle and Talcott found that when from 35 to 125 parts of polytetrafluoroethylene was added to a fluorinated organopolysiloxane containing rubber, a new peak in tensile strength was reached. Their compositions showed improved tensile and tear strengths and good resistance to solvents and fuels.
Polytetrafluoroethylene has been added to compositions containing fluorinated polydiorganosiloxanes to modify the characteristics of the uncured composition. Russel in U.S. Pat. No. 3,192,175, issued June 29, 1965, and Matherly in U.S. Pat. No. 3,630,982, issued Dec. 28, 1971, teach sealing compositions containing poly-3,3,3-trifluoropropylmethylsiloxane gum, reinforcing silica filler, poly-3,3,3-trifluoropropylmethyl siloxane fluid, and polytetrafluoroethylene resin. These sealant compositions require a particular range of viscosity, expressed as plasticity number in order to function properly in their application. The sealing compositions are noncuring. Blizzard and Monroe teach in U.S. Pat. No. 4,010,136, issued Mar. 1, 1977, that the addition of from 0.3 to 1.0 parts by weight of a polytetrafluoroethylene powder, based upon 100 parts by weight of polydiorganosiloxane gum and reinforcing silica filler, to a low durometer curable polydiorganosiloxane composition will reduce the amount of porosity or bubbles formed during the curing step. Their composition requires at least 95 percent of the organic radicals in the polydiorganosiloxane gum to be methyl radicals.
In U.S. Pat. No. 3,132,116, issued May 5, 1964, Wilkus teaches that organopolysiloxanes containing chemically combined organocyanoalkylsiloxane units are extremely tacky and as such are very difficult to process. Such polymers are more oil resistant than conventional polydimethylsiloxane. Wilkus shows that this processing problem is dramatically reduced if from 0.1 to 10 parts of polytetrafluoroethylene is included in the composition.
In his Example 1, Wilkus shows that compositions including polytetrafluoroethylene exhibited improved tensile strength. The resistance to surface reversion on exposure to Mil 7808 oil for 10 hours at 150.degree. C. was also improved.
Donnelly et al. in U.S. Pat. No. 3,669,707, issued June 13, 1972, teach a fixing process in which a particulate thermoplastic toner contacts a silicone elastomer surface. One of the silicone elastomer gums suggested is a polymerized trifluoropropyl and vinyl dimethylpolysiloxane. The preferred fillers are fluorinated resins having surface energies below 30 dynes/cm with polytetrafluoroethylene being shown in the examples. The composition is substantially free of high surface energy fillers such as silica.
None of the above teachings suggest that the retention of physical properties of fluorinated polydiorganosiloxane elastomers after exposure to hot fuel and hot petroleum oil would be improved by adding polytetrafluoroethylene in amounts of 0.8 to 5 parts by weight, per 100 parts by weight of the combination of polydiorganosiloxane gum and reinforcing filler.